The present invention relates to autobaud functions of data communication equipment, and more particularly to a method of performing an autobaud function in data communication equipment utilizing a predetermined state flow machine for processing the bits of at least a portion of a command bit stream.
In modern data communication systems, data terminal equipment (DTE), like personal computers, workstations and the like, communicate data among other DTE's over communication media utilizing correspondingly coupled data communications equipment (DCE), like modems or terminal adapters, for example. Generally, the DTE's are coupled to their respective DCE's by conventional asynchronous, serial communication lines, like RS-232 lines, for example. An information bit stream of digital data is transferred over the communication lines in the form of words or characters, conventionally represented by 7 or 8 coded bits, from the DTE to the DCE for conversion and transmission thereby over the communication media to a remote DCE and associated DTE. Each word or character generally has appended to it at least one parity bit which may be coded in accordance with any one of a number of different parity types. Normally, the receiving DCE does not have prior knowledge of the transmit characteristics, i.e. bit rate, word length, and parity type, of the information bit stream which it is about to receive from the DTE.
For this purpose, an autobaud function may be implemented in the DCE in accordance with some predetermined command standard in order for the DCE to determine such transmit characteristics from attention characters of a command bit stream which precedes the information bit stream of the digital data transfer. Some DCE's respond to a command bit stream which includes certain predetermined ASCII attention characters in sequence in order to determine the word length and parity type of the remaining command bit stream and the succeeding information bit stream.
One presently known autobaud function starts by measuring the length of a start bit of the first attention character which precedes the first attention character of the command bit stream, then samples and rotates each received bit of the first ASCII attention character into a byte, then samples the next two bit intervals for parity type and word length information which is saved. After the first attention character is received, the same operation, except for the measurement of the length of the start bit, is performed for the second attention ASCII character of the command bit stream. Once all of the information of the characters is gathered, a decision on the transmit characteristics of word length and parity type is made and the DCE is configured based on the measured bit rate and word length and parity type determined from the associated attention characters of the command bit stream. In this autobaud procedure, much of the processing to determine the word length and parity type is performed after all of the attention characters of the command bit stream has been received and before the first bit of the remaining command bit stream, which may begin as soon as one half of one bit interval after the last bit of the attention characters has been sampled.
Most modern DCE's use digital signal processors or micro controllers for processing the bit streams transferred thereto from their associated DTE's. Existing processors of the DCEs are generally limited in performance, that is, the processor may be capable of executing the decision making steps of the gathered information of the command bit stream for determining the word length and parity type within the one half bit interval for a bit rate of say 4800 or 9600 bits per second, for example; however, with higher bit rates say on the order of 20-40 kilobits per second, the DCE processor may not achieve a successful autobaud task within the allowable time frame. Of course, it is always possible to substitute a higher performance processor, but this tends to affect the cost of the DCE and ultimately, the market competitiveness thereof.
Accordingly, it is desired to perform the autobaud function in a DCE substantially independent of the transferring data bit rate from an associated DTE without incurring additional significant costs so as not to affect market place competitiveness of the DCE. It would also be desirable to implement the autobaud function at the higher transfer bit rates without having to provide additional special purpose hardware to achieve the autobaud capability at such higher rates.